Abstract
Amblyopia is a neurodevelopmental disorder of vision that occurs when the visual cortex receives decorrelated inputs from the two eyes during an early critical period of development. Amblyopic eyes are subject to suppression from the fellow eye, generate weaker visual evoked potentials (VEPs) than fellow eyes and have multiple visual deficits including impairments in visual acuity and contrast sensitivity. Primate models and human psychophysics indicate that stronger suppression is associated with greater deficits in amblyopic eye contrast sensitivity and visual acuity. We tested whether transcranial direct current stimulation (tDCS) of the visual cortex would modulate VEP amplitude and contrast sensitivity in adults with amblyopia. tDCS can transiently alter cortical excitability and may influence suppressive neural interactions. Twenty-one patients with amblyopia and twenty-seven controls completed separate sessions of anodal (a-), cathodal (c-) and sham (s-) visual cortex tDCS. A-tDCS transiently and significantly increased VEP amplitudes for amblyopic, fellow and control eyes and contrast sensitivity for amblyopic and control eyes. C-tDCS decreased VEP amplitude and contrast sensitivity and s-tDCS had no effect. These results suggest that tDCS can modulate visual cortex responses to information from adult amblyopic eyes and provide a foundation for future clinical studies of tDCS in adults with amblyopia.
Highlights
The results of investigations into the neural basis of amblyopia in humans are broadly consistent with the primate neurophysiology data
We first assessed the effect of a-Transcranial direct current stimulation (tDCS) on pattern reversal Visual evoked potential (VEP)
Our results demonstrate that a single session of anodal tDCS (a-tDCS) can transiently increase VEP amplitude and contrast sensitivity in adult patients with amblyopia
Summary
The results of investigations into the neural basis of amblyopia in humans are broadly consistent with the primate neurophysiology data. Reduced visual cortex excitability measured using phosphene thresholds for single pulses of transcranial magnetic stimulation (TMS) has been observed in patients with amblyopia[38,39], possibly reflecting abnormally high levels of cortical inhibition. We found that a-tDCS of the occipital poles significantly reduced psychophysically measured surround suppression in adults with normal vision[53] This result suggested that a-tDCS might act to transiently reduce GABA mediated inhibition within the visual cortex as had previously been reported for the motor cortex[44]. A-tDCS reduced the cortical response asymmetry to inputs from the fellow and amblyopic eye in these patients Building on this previous work, the aim of this study was to further investigate the effects of tDCS on visual cortex responses and contrast sensitivity in a larger group of adults with amblyopia and controls. We addressed the following questions that were not part of our previous studies: 1) Are changes in amblyopic eye contrast sensitivity induced by tDCS associated with changes in the cortical response to inputs from the amblyopic eye? Cortical responses were measured using monocular pattern reversal VEPs. 2) Do the effects of tDCS on contrast sensitivity and VEP amplitude differ between patients with amblyopia and controls? 3) Do the effects of tDCS on contrast sensitivity and VEP amplitude differ significantly from a sham stimulation control condition?
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